Femoral slideway

ABSTRACT

A femoral slideway and a femoral slideway/femur-size template combination is employed to create an installed knee prosthesis that creates a reduced amount of stress in the collateral ligaments relative to previous prostheses. In one embodiment, the reduced stress is achieved by employing a femoral slideway/femur-size template combination which results in installation of a femoral slideway which has a smaller dorsal-ventral dimension than a corresponding dimension of the femur prior to resection.

DESCRIPTION

[0001] The invention relates to a femoral slideway according to theprecharacterizing clause of claim 1 and to a femoral slideway/femur-sizetemplate arrangement and a knee endoprosthesis system with such afemoral slideway.

[0002] A femoral slideway of this kind is disclosed, for example, in theGerman patent DE 40 41 002 C2. In the surgical technique customarilyused for knee-joint replacement by means of such a femoral slideway, anequal amount of bone is removed from the two condyles of the femur, sothat the anterior or ventral cut is parallel to the posterior or dorsalcut. When the implant is in the position thus defined, the axis ofrotation of the implant no longer coincides with the axis specified bythe arrangement of the collateral ligaments, and this position is notanatomical inasmuch as when flexed, the implant is seated either tootightly on the medial side or too loosely on the lateral side.

[0003] The remedy that was recommended some time ago, namely an outwardrotation of the cutting guide such that in the posterior region lessbone is removed laterally than medially, whereas anterior-laterally morebone is removed than on the anterior-medial side, also presentsdisadvantages, which the construction of the femoral slideway proposedin DE 197 16 879 A1 of the applicant is designed to eliminate. The cruxof this solution is to rotate the anterior or ventral cut in thetransverse plane.

[0004] However, this more recent solution also requires improvement withrespect to optimizing the joint function in cooperation with thecollateral ligaments in particular, especially in order to reduce theload imposed thereon.

[0005] It is thus the object of the invention to disclose a femoralslideway with further improved function, as well as a kneeendoprosthesis system that can be efficiently constructed and employedand has such a femoral slideway as its essential element, and finally anadvantageous arrangement comprising femoral slideway and femur-sizetemplate.

[0006] This object is achieved in its first, foremost aspect by afemoral slideway with the features given in claim 1.

[0007] The invention includes the essential idea that it is advantageousto prepare for a knee-joint replacement by resecting more bone from thefemur dorsally than is replaced by the implant (the femoral slideway).The “diminution” of the femoral slideway thus brought about in thedorsal region, in comparison to the original dimensions of the(resected) condyles or to a femoral slideway fitted in the conventionalmanner, produces an effective reduction of the turning radius of thetibial plate belonging to the prosthesis system and hence reduces theload on the collateral ligaments.

[0008] The decrease in the dimensions of the femoral slideway measuredbetween the outermost, dorsoventrally opposed points on the condyleshell surfaces, in comparison to the previously customary dimensioning,is preferably in the range between 2 and 5%. This is achieved byconstructing the associated femoral slideway/femur-size templatearrangement in such a way that the distance separating one or more pegson the femoral slideway from its dorsal sliding surface is smaller by5-15%, in particular by about 10%, than the corresponding distance bywhich bores in the femur-size template for positioning the pegs areseparated from the contact surface that is to be apposed to the dorsalcondyle surfaces of the femur.

[0009] The distance between the dorsal sliding surfaces and the one ormore pegs on the inside of the femoral slideway is preferably in therange between 24 and 34 mm and in particular is 29 mm, the chosen valueadvantageously being kept constant in a knee endoprosthesis system forcovering a relevant joint-size range.

[0010] Another distinguishing feature of the proposed femoral slidewayis that particular dimensions maintain a largely constant relationshipto one another, regardless of the size of the actual prosthesis. Forinstance, the ratio a:c between the maximal dorsoventral extent and themaximal lateral extent of the femoral slideway is about 0.9±0.02. Thepatellar pit formed between the condyle shells preferably has a depth b,measured from the dorsalmost point on the condyle shells, such that itsratio b:a to the maximal dorsoventral extent of the femoral slideway isin the range between 0.4 and 0.5, in particular is 0.44.

[0011] The patellar pit is thus lengthened in the dorsal direction, as aresult of which the patella can be supported over a large areathroughout its entire functional range of flexion.

[0012] This elongation of the patellar pit, which furthermore increasesin accordance with the anatomy in implants of all sizes, allows for thefact that the patello-femoral contact surface in conventional femoralslideways has a relatively small bearing area. That is, in the region inwhich the patella leaves the trochlea and enters the intercondylarfossa, conventional femur components provide support only in theperipheral regions.

[0013] Furthermore, in the proposed femoral slideway the condyle shellsare somewhat more strongly rounded in cross section (coronal section)than is the case in conventional femoral slideways. This modificationwas undertaken in the interest of improving the fit to the special tibiainsert that belongs to a knee endoprosthesis system, but which is notwithin the scope of the invention.

[0014] The back surface of the femoral slideway, in one advantageousembodiment, bears a two-component Ti coating produced in a vacuum plasmaprocedure, consisting of a relatively thin, dense base layer and aseveralfold thicker, open-pored cover layer. The dense base layer allowsthe femoral slideway, which for example consists of CoCrMo, to becomecompletely sealed to the bone and, because it makes contact with thesubstrate over a large area, increases the stability of adhesion.

[0015] The open-pored and very rough surface of the cover layer providesideal conditions for the growth of bony substance onto and into thecarriage, producing a quasi “3-D interlocking” that can transmit pullingforces as well as pressure and transverse forces.

[0016] Additional advantages and useful features of the invention willbe apparent from the subordinate claims and the following description ofan exemplary embodiment with reference to the figures, wherein

[0017]FIG. 1 is a view (from proximal) of a femoral slideway accordingto one embodiment of the invention,

[0018]FIG. 2 shows the femoral slideway according to FIG. 1 in mediansection (sagittal section),

[0019]FIG. 3 is a plan view of an embodiment of a femur-size template,

[0020]FIG. 4 is a side view of the latter,

[0021]FIGS. 5a, 5 b show a conventional arrangement of a femoralslideway on a femur in comparison to an arrangement proposed here, and

[0022]FIGS. 6a, 6 b show scanning electron micrographs of a crosssection of a conventional layered structure and of an embodiment of thelayered structure proposed here for the back-surface coating of afemoral slideway.

[0023] In FIGS. 1 and 2 the femur component 10, called a femoralslideway, of a knee endoprosthesis is shown. The femoral slideway 10comprises two convexly curved condyle shells 11, 12 and a patellarshield 13, which connects the two condyle shells 11, 12 rigidly to oneanother.

[0024] The condyle shells 11, 12 and the patellar shield 13 in theirinteriors define anterior and posterior fitting surfaces 14, 15 thatcorrespond to a femoral ventral and dorsal cut, respectively, and areassociated with a ventral and a dorsal saw-cut surface produced at thedistal end of the femur when the latter was resected for fitting of thefemoral slideway. The convex outer shape of the condyle shells 11, 12specifies dorsal sliding surfaces 11 a, 12 a in the posterior region,over which the corresponding surfaces of the tibia insert slide when theknee endoprosthesis is flexed. The patellar shield 13, which is recessedwith respect to the convex outer surfaces of the condyle shells 11, 12,defines a so-called patellar pit 16, within which there is supported apatella component 17 of the knee endoprosthesis, which is indicated by adashed outline in FIG. 2 and does not belong to the femoral slideway 10.

[0025] To assist anchoring and central placement of the femoral slideway10 on the femur, on the inner surface of the femoral slideway two pegs18, 19 are formed, the long axis of which is substantially parallel tothe posterior fitting surface 15. These pegs project into holes in thefemur, which have been drilled in the appropriate positions with the aidof a corresponding drilling template (see below), and this engagementgives the attachment of femoral slideway to bone greater stability thanis provided by the fitting surfaces alone.

[0026] To ensure that the the femoral slideway will function optimallyas a replacement for destroyed sliding surfaces on the femur, theconstruction must reflect as accurately as possible the anatomicalarrangements and dimensions, but also within the scope of the inventionincludes a specific modification that will now be explained.

[0027] One of the relevant dimensions of the femoral slideway 10 is themaximal anterior-posterior or dorsoventral extent of the condyle shells1, 12, a distance labelled a in FIG. 1. Another relevant dimension isthe maximal lateral extent of the femoral slideway, i.e. the distancebetween the most lateral point on the lateral condyle shell 11 and themost medial point on the medial condyle shell 12, which in FIG. 1 islabelled c. Also significant is the distance from the outermostposterior point on the dorsal sliding surfaces 11 a, 12 a of the condyleshells 11, 12 to the posterior bounding edge of the patellar shield 13,which in FIG. 1 is labelled b. A final significant distance is thatbetween the outermost posterior points on the dorsal sliding surfaces 11a, 12 a and the long axis of the pegs 18, 19 (which lie in one and thesame coronal plane), labelled d in FIG. 2. In the exemplary embodimentdescribed here, the ratio a:c is 0.9 and the ratio b:a is 0.44. Ongrounds of biomechanics and surgical technique, it has proved useful tomake the distance d (between sliding surface and peg axis) uniform forall sizes of femoral slideway used in a knee endoprosthesis system. Inthe present case, this distance is 29 mm.

[0028] To determine the correct femoral slideway size, a femur-sizetemplate 20 shown in FIGS. 3 and 4 is used. This comprises a basic part21 with two flanks 22 and 23, each of which ends in a contact section 22a, 23 a that is bent at a right angle and is apposed to the condyles ofa femoral bone that is to be fitted with a femoral slideway (FIGS. 1 and2).

[0029] In the middle of the basic part 21 a measurement tongue 24, bentat an angle in two places, is mounted so that it can be displaced in adirection perpendicular to the plane in which the contact sections 22 a,23 a lie. The measurement tongue 24 is marked with a scale 25, whichindicates the maximal anterior-posterior extent of the head of thefemur, i.e. the condyles, and thus indicates to the doctor the requiredsize of the implant. In the basic part 21 of the femur-size template 20two peg-hole bores 26, 27 are provided, which—in accordance with asupplementary drilling-template function of the femur-sizetemplate—assist the positioning of peg-holes in the femur so that theycorrespond to the pegs 18, 19 of the femoral slideway 10 as shown inFIG. 1. The axes of the peg-hole bores 26, 27 are separated by adistance e from the contact surfaces of the contact sections 22 a, 23 a.This distance—along with the distance d between the sliding surfaces andpegs on the femoral slideway 10 itself (cf. FIG. 2)—is an additionalrelevant dimension in the concrete implementation of a kneeendoprosthesis, for the following reason:

[0030] So that the above-mentioned peg-holes—which serve not only toposition the implant but also to position the cutting guides used toproduce the various saw cuts on the femur—can be drilled into the bone,drill bushes (not shown) are inserted into the peg-hole bores 26, 27.

[0031] It has proved advantageous, in particular from the viewpoint ofreducing the load on the collateral ligaments during flexion of theartificial knee joint, to resect more bone dorsally on the femur thanwill be replaced there by the thickness of the dorsal parts of thecondyle shells. For this reason the distance e is made larger than thecorresponding distance d (FIG. 2). In the preferred embodiment therelative distance reduction, i.e. the quantity (e−d)/d, is about 10%.

[0032] The effect thus achieved can be seen in FIG. 5, where a sketchrepresenting the conventional way of attaching a femoral slideway 10′ toa femur F′, shown in FIG. 5a, is compared with the representation inFIG. 5b of the arrangement proposed here. The anterior-posterior extentof the femoral slideway 10 in FIG. 5b, mounted on a femur F resectedfurther in the dorsal region, is smaller by the amount (e−d) than in theconventional implant 10′.

[0033] Because the distance e is permanently specified by the femur-sizetemplate, which is used for all implants regardless of their size, andaccordiing to what has been stated above the distance d in theembodiment of the femoral slideway is preferably kept constant for allimplant sizes, the geometric relations will be slightly different forimplants of different sizes. This is acceptable, however, in view of theadvantages for manufacture and manipulation that such a system brings.

[0034]FIG. 6b shows—in comparison to a conventional femur-carriagecoating as shown in FIG. 6a—the appearance in the scanning electronmicroscope of a cross section through a two-component titanium-coatingconstruction consisting of a dense base layer G, about 50 μm thick, andan open-pored cover layer D averaging about 250 μm thick, on a CoCrMosubstrate S. Although the thickness and average roughness of the coatingaccording to FIG. 6b, which is applied by a vacuum plasma process, arecomparable to those of the known, sprayed-on coating according to FIG.6a, it should be emphasized that the former has a more open-poredstructure and a considerably reduced number of interface defects(indicated in both pictures by vertical arrows).

[0035] Implementation of the invention is not limited to the exemplaryembodiment described above, but can also incorporate modifications,which in particular include departures from the specified dimensions andratio values. LIST OF REFERENCE NUMERALS 10, 10′ Femoral slideway 11, 12Condyle shells 11a, 12a Dorsal sliding surfaces 13 Patellar shield 14Anterior fitting surface 15 Posterior fitting surface 16 Patellar pit 17Patella component 18, 19 Peg 20 Femur-size template 21 Basic part 22, 23Flanks 22a, 23a Contact sections 24 Measurement tongue 25 Scale markings26, 27 Peg-hole bores a, b, c, d, e Distances A-A Plane of section DCover layer F, F′ Femur (shaped) G Base layer S Substrate

What is claimed is:
 1. A method of preparing a lower extremity of afemur and implanting a femoral slideway thereon, the method comprising:providing a femoral slideway having at least one peg adapted forinsertion into a hole in the femur; locating a point on a lowerextremity of a femur, said point being located at a pre-determineddistance from a plane tangent to dorsalmost points of lateral and medialcondyles of the femur; drilling a hole at said point; selecting afemoral slideway having a dimension defined by a perpendicular distancebetween a longitudinal axis of a peg extending from the slideway and aplane tangent to a dorsal sliding surface furthest away from the peg;wherein said pre-determined distance is larger than said dimension. 2.The method of claim 1, further comprising resecting bone material fromthe femur and fitting the slideway onto the femur by inserting the peginto the hole.
 3. The method of claim 1, wherein said pre-determineddistance is about 5% to about 15% larger than said dimension.
 4. Themethod of claim 3, wherein said pre-determined distance is about 10%larger than said dimension.
 5. The method of claim 1, wherein saidlocating is performed before said drilling.
 6. The method of claim 1,wherein said selecting a femoral slideway further comprises selecting aslideway with a dimension between a dorsal sliding surface and a ventralsliding surface which is a previously-determined percent of a distancebetween a dorsal-most condylar surface and a ventral-most condylarsurface of the femur before said resecting.
 7. The method of claim 6,wherein said percent is about 2 to 5%.
 8. The method of claim 1, whereinselecting further comprises selecting a femoral slideway from a group ofslideways, wherein all of the slideways in said group have substantiallyequal peg-to-sliding-surface dimensions as defined by a perpendiculardistance between a longitudinal axis of a peg extending from theslideway and a plane tangent to a dorsal sliding surface furthest awayfrom the peg; and wherein each slideway in said group has a differentdorsal condyle-to-ventral condyle distance.
 9. A method of preparing alower extremity of a femur for implantation of a femoral slideway, saidmethod comprising: determining a location for a pair of holes to bedrilled in a lower surface of a femur bone by indicating points at apreviously-determined distance from a dorsalmost point of a condyle ofsaid femur; wherein said previously-determined distance is about 5 to15% larger than a distance between a peg and a dorsal sliding surface ofthe femoral slideway to be implanted on said femur bone; drilling a holein a lower surface of a femur bone at said points; resecting bonematerial from said femur, wherein said resecting includes removing morebone material from a dorsal side of the femur than will be replaced bythe slideway implanting the slideway on the femur by inserting the peginto the hole.
 10. The method of claim 9, wherein saidpreviously-determined distance is about 10% larger than a distancebetween a peg and a dorsal sliding surface of the femoral slideway to beimplanted on said femur bone.
 11. The method of claim 10, wherein thedistance between the peg and the dorsal sliding surface of the femoralslideway is between about 24 mm and about 34 mm.
 12. The method of claim11, wherein the distance between a peg and a dorsal sliding surface ofthe femoral slideway is between about 29 mm.
 13. The method of claim 10,wherein said previously-determined distance is between about 26.4 mm andabout 37.4 mm.
 14. The method of claim 13, wherein thepreviously-determined distance is about 32 mm.
 15. The method of claim9, wherein said previously-determined distance is between about 30.45 mmand about 33.35 mm.
 16. A method of implanting a femoral slideway on afemur, said method comprising: providing a slideway comprising twoconvexly curved condyle shells rigidly connected to one anotheranteriorly by a patellar shield, wherein outer surfaces of the condyleshells define dorsal sliding surfaces of the femoral slideway, whereinthe femoral slideway has on an inner surface at least one peg, whereinbetween a long axis of the peg or pegs and a point on a dorsal slidingsurface furthest away therefrom a sliding-surface-to-peg distance isdefined, and wherein an overall extent of the femoral slideway in ananterior-posterior direction is smaller than a corresponding originaloverall extent of the condyles of a femur to which the femoral slidewayis fitted; resecting bone material from a femur, wherein said resectingincludes removing more bone material from a dorsal side of the femurthan is replaced by the slideway; providing at least one hole in saidfemur to receive said at least one peg of said slideway and to secureand locate said slideway on said femur; and fitting said slideway ontosaid femur.
 17. The method of claim 16, wherein said resecting includesremoving about 10% more bone material from the dorsal side of the femurthan is replaced by the slideway, as measured linearly between saidholes and a dorsalmost surface of said femur.
 18. The method of claim16, further comprising positioning a femur-size template on said femurto determine the position of said at least one hole in the femur. 19.The method of claim 18, wherein said femur-size template comprises adorsal condyle-engaging surface and at least one bore separated by apre-determined distance.
 20. The method of claim 19, wherein saidpre-determined distance is about 5 to 15% larger than a perpendiculardistance between said at least one peg and said dorsal sliding surfaces.21. The method of claim 20, wherein said pre-determined distance isabout 10% larger than a perpendicular distance between said at least onepeg and said dorsal sliding surfaces.